Questions tagged [tcp]

For questions about Transmission Control Protocol (TCP), a transport protocol used to encapsulate data across a network for reliable communication.

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ports and security

I'm thinking about how the "port" operates for the network connections and tried reading the CCNA cert. guide to understand the details, however, I need to clarify doubts: does occupying all the TCP/...
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Finding MSS for a remote connection that uses tunnelling

I'm working on a codebase where we're trying to test network health by verifying if we can ping the remote site having payload = MSS without any packet fragmentation (using -df flag). Typically, for ...
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Not a four-way handshake in Cisco Packet Tracer?

TCP connection termination is performed by the four-way handshake, as shown below (the image is taken from here). I tried to verify it on Cisco Packet Tracer. With the following topology, I captured ...
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Pros and cons of the idea of parallell data transmission

I am learning the basics of internet protocols and related principles. I recently performed experiments comparing UDP and TCP. But when I did that (using the ttcp program) it was only one process and ...
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2 answers
96 views

Why the three-way handshake use so short time accomplish it?

I use the Wireshark captured the packages: in there you see the number 1594-1596, the three-way hankshake only use 0.0002s(0.2ms) it complete it. but when I in the server(103.215.9.27) ping the ...
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768 views

what is the definition of out degree and in degree in computer networks?

in wikipedia we have a good definition for in degree and out degree in a graph: " The degree of a node is the number of connections or edges the node has to other nodes. in-degree is the number of ...
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Why the TCP client haven't receive the ACK response, it send the FIN package?

I use the WireShark captured the TCP connect. you see the three-way handshake(1-3) and the four-way handshake(5-8): But, from the frame list, I have a question about TCP. You know the TCP should ...
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Why the seq number set to random, there will be safer in TCP connect?

From the TCP document I have read this: First, client sends a TCP packet with_ SYN=1, ACK=0 and ISN(Sequence Number)= 5000_. Do not forget, sequence number is random and it could be between 0 to 4,...
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is there any limit/role for `syn` initial number and `seq` number of TCP three-way handshake?

We know the TCP three-way handshake, I want to know is there any limit/role for syn initial number and seq number? I hear about the number set random can protect server, who can help talk about this?
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60 views

Is it possible to re-transmit only the last packet at TCP flow?

I try to understand the TCP behavior, especially re-transmission. I did a small experiment and find the curious thing. The only last packet is re-transmitted. Why do this happen? I think if the ...
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What's the hook and double hook meaning in wireshark?

What's the hook and double hook meaning in wireshark? you see the two snapshots up there, when I select #7 the #6 will be hooked. when I select the #8, the #5 will be double hooked. What's the ...
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The reason a TCP connection generate retransmission

My question is why a TCP flow make a re-transmission when a network has enough link bandwidth. For finding a cause, I used a wireshark. I got the below captured at a host side (10.0.0.1) 85968 47....
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2 answers
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If you're behind a firewall and all the ports are blocked, how is your browser able to access the internet? [closed]

I'm behind a router and I'm able to run game servers on my home computer after forwarding ports or using the DMZ feature on the router. However, I still don't understand the underlying technology as ...
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0 votes
1 answer
2k views

TCP cwnd is increasing when congestion occur

I am a newbie network engineer. I did a small experiment with iperf3. But I run into a small problem. (1) My question. I think TCP flow do not act properly despite of its congestion control. The flow ...
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1 answer
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How TCP find the correct application?

I am learning how TCP and application communicate with each other. I am not sure if my understanding is correct, I will use below example to illustrate my understanding and want to ask some questions. ...
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What technique is my ISP using to censor web traffic? [closed]

BBC news is censored in the country where I live. Out of interest I decided to briefly look at how this is done. First of all they apparently MITM DNS - if I look up BBC domains on Google's 8.8.8.8 I ...
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Why do transport layer do data chunking. If there is fragmentation in Network Layer

I just curiously wanted to know about the question regarding L4, L3 Data Chunks, IP Fragmentation : Data chunking happened at Layer 4(Transport, TCP), and Fragmentation at Layer 3(Network, ipv4) ...
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How can I accomplish VPN across Dual Wan connections?

How can I accomplish VPN across Dual Wan connections in a optimal way? Idea is to make: Site to site VPN. Make single connection inside the VPN to use full bandwidth of both wan connections using (...
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4 votes
1 answer
551 views

TCP connections hang during handshake [closed]

For the last few days, connections originating behind my LAN's NAT have been having issues completing the TCP handshake (i.e. web pages only load after hitting F5 a few times, network applications ...
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6 votes
3 answers
3k views

Sending less data than the TCP buffer could take

The accepted answer of this question explains: When you send data, your TCP buffers it. So if you send a character it won't send it immediately but wait to see if you've got more. But what if I ...
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Juniper EX Switch sending TCP reset on SSH connection randomly

So as stated in Title I'm observing that one of our switch randomly send RESET flag in TCP and ends the ssh connection in the middle and sends and Window size of 0 as well. Randomly it refuses to ...
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2 votes
1 answer
753 views

Does HTTP proxy sends its own TCP packets or bypasses clients packets?

Need help to understand what's happening on TCP level when http proxy is used. When client wants to open a website through a proxy, first connection is established between client and proxy. Then ...
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0 votes
1 answer
498 views

Sending TCP packet to the other device without connection

I think the sending packet to other device works like the following. 1. The server device listens with port. for example 80 2. The client connects to the server devices. for example, the port of ...
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28 votes
7 answers
7k views

Why isn't UDP with reliability (implemented at Application layer) a substitute of TCP?

TCP provides reliability at transport layer while UDP does not. So, UDP is fast. But, a protocol at application layer can implement reliable mechanism while using UDP. In this sense, why isn't UDP ...
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TCP sequence numbers in Actual Transmission vs. in Wireshark Capture

Whenever, we do a wireshark capture and Follow--> TCP Stream, we see the sequence numbers as fixed values. In reality, if this were the case, the intruder can easily predict the transmission patterns....
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Response time of TCP congestion control

I would like to know about the overall latency of the TCP congestion control for adapting to varying network conditions. Specifically, my questions are: How fast can the system react after a change ...
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2 answers
6k views

How to measure max bandwidth with Wireshark?

I'm trying to measure the max bandwidth usage of my LAN with Wireshark. I´ve done several packet captures but I don't know how to obtain the max bandwidth usage. Any Idea? Thank you in advance
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Why the Ack number is bigger than the last Seq number?

Why the Ack number is bigger than the last Seq number? you see the Ack response for each two TCP packages. but there I have a question, you see the Ack response for Seq=19997, Seq=22893 and Seq=...
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1 vote
1 answer
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Dropping packets between router and firewall without firewall rule

I have a user that is trying to access his company network resources on his home network. He is able to access SMB shares and other resources, but he is unable to access their AS400 on any computer in ...
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1 vote
1 answer
439 views

Wireshark captures TCP packet with no flag [closed]

In our network, I captured a TCP packet by Wireshark that has no flag set. What's that mean? Is it normal?
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3 votes
2 answers
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Behaviour of TCP SACK

I will ask the question based on the following scenario. Assume I have to send 10 packets using TCP using a window size of 5. I send 1, 2 and receive ACK for them, I send 3 and the ACK is lost, I send ...
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1 vote
1 answer
970 views

Why server cannot send data in TCP three-way handshaking?

In the three-way handshaking, only from the ACK packet we can send data. Why cannot the server send data with SYN-ACK packet? I understated why a client cannot send data to a server (preventing, for ...
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2 votes
3 answers
1k views

What's the use of an acknowledgement number without the ACK flag set?

Isn't it redundant if the ACK flag in the header isn't also on? Do you ever mix an acknowledgement with a normal message - send data and acknowledge a message in the same TCP segment? I feel like ...
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8 votes
6 answers
5k views

Could IPv6 make NAT / port numbers redundant?

From what I can tell, each process could get its own IP address, with loads of IP's to spare. What would the drawbacks be? Essentially, your host would become a router and each process is a host (in ...
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1 vote
2 answers
779 views

TCP Three Way Handshake?

I have the following query about the tcp handshake. To establish a connection the server must send a packet with SYN flag to 1 Then the server replies with a SYN,ACK flag to 1 , and afterwards the ...
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2 votes
1 answer
412 views

Why does TCP need segments if the network layer automatically breaks things up into fragments?

I'm trying to better understand the difference between IP fragments and TCP segments (https://www.shubhsblog.com/tech/difference-ip-fragmentation-tcp-segmentation.html) and there's one thing I'm not ...
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2 votes
2 answers
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If an old FIN is delivered, will TCP terminate the new connection?

Two programs use a TCP connection (3-way handshake) to open, communicate, terminate the connection and open a new connection. If a FIN message sent to shut down the first connection is duplicated and ...
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3 votes
0 answers
82 views

Why does TCP exit out of slow-start faster than expected? [closed]

For context, I am testing a client-server TCP flow, where the client makes a request for a file, and the server sends the file using regular TCP streams. Both servers are using the "Cubic" ...
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2 votes
2 answers
575 views

Transferring data speed of Fast Ethernet

I am wondering that between 2 hosts in LAN, when i send one file which weights 1Mb versus 1 file weights 0.3Mb from the first host to the second host using TCP/IP protocol, would the latter come first?...
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0 answers
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Why do I not see slow start in this TCP packet capture? [closed]

I am looking at the packet flow between a server and a client (which is downloading a file from the server). As you can see in the packet capture below (with IP addresses omitted), after the initial ...
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-2 votes
1 answer
906 views

How to send data from one PC to another PC over internet? [closed]

I'm basically a developer, and this is the first time i'm trying to develop a chat application over internet without any servers in between. It's easy to send data from PC to server and then to ...
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1 vote
3 answers
566 views

Is IP an abstraction

If packets "hop" from network device to network device across networks using MAC addresses, resolved by ARP, does this suggest that IP is essentially an abstraction? By abstraction, I mean, the ...
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10 votes
4 answers
16k views

Can anyone explain to me the difference between the PSH and URG flags in TCP segment?

According to me when PSH flag is set, it indicates that data will have to be sent immediately to the wire and on receiving side the data has to be pushed to the application immediately. When URG flag ...
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4 votes
2 answers
301 views

Is a TCP segment with FIN but not ACK ever encountered today?

A fin-scan involves sending a TCP segment with FIN and no other flags other than (possibly) PSH and/or URG. Are such packets ever legitimately encountered in 2019? Note that in Is a FIN only segment ...
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-1 votes
1 answer
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why can't we use the sequence number to distinguish them since every packet has a different sequence number? [closed]

It is a question from Networking stanford course. A server typically listens for TCP connections on a particular port (e.g. 80 for HTTP or 22 for SSH). Which fields can the server use to ...
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4 votes
1 answer
302 views

Is an RST only segment ever legitimate?

Is a TCP segment with RST but no ACK ever legitimately encountered today? While some kernels emit such an RST when a SYN segment is sent "illegitimately" from themselves (e.g., nmap half-syn, scapy, ...
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3 votes
1 answer
528 views

How is SampleRTT measured once every RTT, without measuring every segment?

According to the RFC, SampleRTT is measured once every RTT. My text: Computer Networking: A top-down approach (6ed By: Kurose & Ross) also states, that: Instead of measuring a SampleRTT for ...
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4 votes
2 answers
354 views

Is it correct to assume that ordering of frames at switch level is not important?

My understanding is that if the payload of the frame contains any data that needs to be reassembled, at least for TCP payload, it can be done at the OS level, once all the packets are received and ...
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2 votes
1 answer
61 views

Is WRED configured for TCP traffic only?

I am looking out for a practical usage of WRED. After reading/searching I found that it is more suitable for TCP traffic. When the queue is full, all the TCP session at that time will start with slow ...
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4 votes
1 answer
689 views

TCP - correlation between ACKs and receive window

EDIT: I think i get it now. So a simple sliding window mechanism basically assumes the correctly received data is processed immediately. Therefore an acknowledgement implies the data was received AND ...
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